outlook 2003 - sudden email appearences

In outlook you can choose to display messgaes from today, last week, 2 weeks ago etc.....or not by clicking on the plus sign (shows all emails)/minus sign (shows just a heading) alongside each group. I seem to be getting a strange issue in that if I do not have any mail displayed from this week, but if I open up (hit the plus sign) for last weeks I get a sudden inflow of e-mails for this week..